The present invention has the objective of providing an electro-acoustic transducer drive circuit suitable for driving a transducer such as a miniature piezoelectric buzzer to emit musical notes, each note having a damped waveform envelope (i.e. decaying gradually from an initial peak amplitude), and provides the capability for producing a plurality of such notes of different frequencies mutually superimposed to form musical chords.
In the prior art there have been various proposals for drive circuit means for producing sounds resembling musical tones or notes from devices such as the miniature piezoelectric buzzer commonly used in miniature electronic devices such as electronic wristwatches. However for various reasons which will be described in detail hereinafter with reference to prior art embodiments of such circuits, it has not been found possible with such prior art circuits to provide more than a very limited range of output acoustic frequencies. In addition, a relatively large number of circuit elements are generally required to form a tone or note of a specific frequency. Due to these considerations, it has not been possible to form a drive circuit suitable for the production of musical chords (i.e. mutually superimposed musical notes or tones) which is capable of being incorporated within an integrated circuit chip such as is in widespread use at present in miniature electronic devices such as electronic wristwatches, and in particular within a MOS transistor type of integrated circuit.
The present invention discloses a drive circuit which overcomes the disadvantages of such prior art circuits, being fully capable of implementation by a relatively small number of elements which can readily be provided within a MOS IC chip, with the frequency and envelope shape of each note produced being determined by digital signals which can be readily derived from the timebase signal generating circuit and frequency divider circuits generally utilized in a miniature electronic device such as an electronic wristwatch, and providing the capability of combining any number of musical notes of different frequencies to form musical chords.